An outphasing amplification technology is known. An outphasing amplifier that executes an amplification process by the outphasing amplification technology is also referred to as a “constant envelope amplifier”.
An outphasing amplifier performs vector decomposition to decompose an input signal into two signals of two constant amplitudes. That is to say, an outphasing amplifier performs amplitude phase conversion to convert an input signal into two signals. The outphasing amplifier performs D/A conversion on the two signals that have been subjected to vector decomposition, and amplifies the two signals. Then, the outphasing amplifier combines the two amplified signals. By executing such a process, the efficiency of the amplifier can be enhanced.
There is known a constant envelope linear amplifier in which the linearity of the amplifier is enhanced, in respect to a constant envelope high efficiency amplifier for enhancing the efficiency of the amplifier (see, for example, patent document 1).
Patent document 1: Japanese Laid-Open Patent Publication No. H3-232306
The outphasing amplifier performs amplitude phase conversion to convert an input signal into two signals of two constant amplitudes.
When an input signal is subjected to amplitude phase conversion to be converted into two signals of two constant amplitudes, the frequency band of the two signals obtained by amplitude phase conversion increases.
FIGS. 1A through 2C illustrate an amplitude phase conversion process.
FIG. 1A illustrates an example of an input signal. The input signal is expressed by a sine wave, and with the passage of time, the amplitude changes as indicated by (1), (2), (3), (4), and (5).
FIG. 1B illustrates an example of time transition of a signal on a phase plane. In the phase plane, it is assumed that the X axis is an I phase (In Phase) and the Y axis is a Q phase (Quadrature Phase), and the signal changes on the X axis.
FIG. 1C illustrates an example of a frequency component of a signal. The signal changes on the I phase, and therefore the frequency component of the signal does not become wide.
FIG. 2A illustrates an example of a process of performing an amplitude phase conversion process on an input signal. When amplitude phase conversion is performed, the amplitude and the phase change in the order of (1), (2), (3), (4), and (5).
FIG. 2B illustrates an example of time transition of a signal on a phase plane. In the phase plane, it is assumed that the X axis is an I phase and the Y axis is a Q phase, and the signal changes on a plane expressed by the X axis and the Y axis.
FIG. 2C illustrates an example of a frequency component of a signal. The signal changes on a plane expressed by the I phase and the Q phase, and therefore the frequency component of the signal becomes wide.
FIG. 3 illustrates an example of an amplifier circuit 10.
The amplifier circuit 10 includes amplitude phase converters 121 and 122, D/A converters 141 and 142, amplifiers 161 and 162, and a combiner 18.
Input signals are subjected to amplitude phase conversion by the amplitude phase converters 121 and 122. The input signals that have been subjected to amplitude phase conversion by the amplitude phase converters 121 and 122 are subjected to D/A conversion by the D/A converters 141 and 142.
The signals that have been subjected to D/A conversion by the D/A converters 141 and 142 are amplified by the amplifiers 161 and 162. The signals that have been amplified by the amplifiers 161 and 162 are combined by the combiner 18. The signals that have been combined by the combiner 18 are output.
FIG. 3 (A) illustrates a spectrum of a multicarrier modulation wave, as an example of an input signal input to the amplifier circuit 10. By being subjected to amplitude phase conversion by the amplitude phase converters 121 and 122, the frequency band of the input signal becomes wide. Therefore, the frequency band of the input signals converted to analog signals by the D/A converters 141 and 142 becomes wide. FIG. 3 (B) illustrates a spectrum of an input signal converted into an analog signal by the D/A converter 142. The spectrum of an input signal converted into an analog signal by the D/A converter 141 is also substantially the same as that of FIG. 3 (B).
To convert a signal of a wide band, the D/A converter needs to have a certain level of accuracy. That is to say, the D/A converter needs to have high resolution performance. Furthermore, to process signals of a wide band, the D/A converter needs to have high processing speed. That is to say, the sampling rate of the D/A converter needs to be high. Accordingly, a D/A converter having high resolution performance and a high sampling rate is needed, but it is difficult to prepare such a D/A converter.
Furthermore, when the amplifiers 162 and 162 have different properties, it is not possible to implement zero output. When it is not possible to implement zero output, it is not possible to use a predistorition method as a distortion compensating method.